138 LECTURE VII. 



been but little studied. It may possibly be due to a hydrolytic decom- 

 position. 



Another group of albuminoids is included under the name of keratins. 

 They comprise the so-called " horn substance," and are found, as such, 

 in the hair, feathers, nails, hoofs, horns, etc. N euro-keratin, which forms 

 a part of the sheath of the medullary nerves, belongs to this class. The 

 keratins are noted for their insolubility in water, dilute acids and alkalies. 

 They have a high percentage of sulphur. Ovokeratin, in the membranes 

 surrounding the eggs of many animals, is closely related to the keratins. 

 Gorgonin, constituting the foundation of the axial skeleton of coral 

 (Gorgonia cavolini), is also included. It is especially noted for its high 

 percentage of iodine. 



The elastin bodies form a group by themselves. They predominate in 

 the formation of the elastic tissues. Elastin is generally prepared from 

 the Ligamentum nuchce of the ox. 



Fibroin, obtained from the threads of silk-worms, is the best known 

 albuminoid. It is characterized by a very low percentage of di-amino 

 acids. We shall return to this substance later. 



Spongin, forming the framework of sponges, and conchiolin, the basis 

 of the skeleton of the mussel, are included among the albumoids. Amy- 

 loid, to whose content of chondroitin-sulphuric acid we recently referred, 

 is also assigned to this group. It occurs only under pathological condi- 

 tions. It is found, on the one hand, in the form of small kernels in the 

 brain (the so-called Corpora amylacea), and then, again, as large deposits 

 in the parenchyma of many organs. This is called an amyloid degenera- 

 tion. The cause of its formation is but little known. 



We must finally consider the group of the albumoids. It includes 

 the most varied albuminous substances, about whose constitution nothing 

 is at present known. To this class belong the Membrance proprice of many 

 glands, sarcolemma, osseo-albumoid, chondro-albumoid, the albumoid of 

 lentils, the elementary matter of Chorda dorsalis, ichthylepidin (occurring 

 in the scales of fishes), the horny layer in the crop of birds, reticulin (which 

 composes the reticular tissues of the intestinal mucosa), and many other 

 analogous substances. 



It is impossible, at present, to give any further exact account of the 

 various representatives of the albumoids. The fact that they are 

 obtained with difficulty, and, more especially, that it is almost impossible 

 to purify them, has made any exact study of the class up to the present 

 time entirely futile. Observers have concerned themselves chiefly with 

 an investigation of various physical properties of individual proteins of 

 this group, and especially to the action of ferments upon them. Their 

 practical indigestibility, has made it possible to remove the ordinary pro- 

 teins from them. The great predominance of albuminous substances, not 



